This invention relates generally to a commutator for an electric motor and to a device for measuring the rotation of an electric motor rotor shaft of the commutator. In particular, the invention applies to an electric motor designed to drive motor vehicle equipment, such as a window regulator, a sunroof or a seat operation motor, for example.
French Patent No. 2,814,868 discloses an electric motor including a commutator.
A conventional commutator 10 is illustrated in FIG. 1. The commutator 10 includes a support ring 11 having a tubular shape made of an insulating material, such as plastic. The support ring 11 is surrounded by a plurality of metal commutator segments 18 arranged over the entire periphery of the support ring 11. The support ring 11 can include notches which receive the metal commutator segments 18. The notches can define a rib 14 between each pair of adjacent metal commutator segments 18. The metal commutator segments 18 are consequently perfectly insulated from each other by the central support ring 11 and the ribs 14. The support ring 11 can, for example, be directly molded in plastic with an appropriate shape or can be tooled. The metal commutator segments 18 also include hook members 13 formed at one end of each metal commutator segment 18. The hook members 13 allow subsequent connection of the commutator segments 18 to the rotor windings.
An electric motor generally includes a stator frame and a rotor mounted rotatably in the stator. The motor also includes windings integral with the rotor shaft, and each winding is electrically connected to two diametrically opposed segments of the commutator 10 by the hook members 13.
The commutator 10 is generally mounted in a commutator housing integral with the stator. The housing includes a support having a central opening designed to receive the commutator 10 and to allow the rotor shaft to pass through the housing support. The housing support is made of an insulating material, for example plastic.
The commutator housing also includes at least one pair of brushes diametrically opposed relative to the commutator 10 to make contact with the segments of the commutator 10 during the rotation that is linked to the rotor shaft. The brushes are electrically connected to a supply of electric current and supply the stator windings with current.
Additionally, in an electric motor, it is necessary to control the rotor shaft speed and to know, at any time, the speed of rotation, the direction of rotation and the angular position of the rotor shaft. This information is particularly necessary for the electronics generally associated with a window regulator motor that performs applications associated with the window regulator, such as an anti-trapping function, a resume raising function, a slowdown-at-end-of-travel function, or the like.
A Hall effect sensor is used to determine the speed of rotation of the motor rotor shaft. A magnetic ring is arranged on the rotor shaft and provides a rotating magnetic field integral with the rotor shaft rotation. The Hall effect sensor is generally arranged near the magnetic ring to determine the rotor shaft speed of rotation from the frequency of alternation of the magnetic field. The Hall effect sensor can also be arranged far from the magnetic ring, for example, on a printed circuit board of the electronics associated with the electric motor. Flux guiding members then lead the magnetic flux from the rotor shaft ring to the Hall Effect sensor. This solution is disclosed in EP 0,891,647.
To determine the direction of rotation of the rotor shaft or the angular position, two Hall effect sensors are used to measure the magnetic flux at separate points of the magnetic ring arranged on the rotor shaft. Although Hall effect sensors provide reliable measurements, they are expensive and complicate motor design.
Apart from the cost of the sensor or sensors, it is also necessary to mount a magnetic ring on the rotor shaft and to optionally provide flux guiding members, which is also a burden on motor production.
It is also necessary to ensure that the magnetic flux employed for measuring the rotor shaft rotation does not interfere with operation of the motor and that the electric components of the motor (commutator brushes, stator windings or otherwise) do not interfere with the magnetic measurements.
There is consequently a need for a device for measuring the rotation of an electric motor shaft which is simplified and yet reliable.